Impregnation of cellulosic material



* Patented Jan. 6 1948 2,434.1 mnremrron or CELLULOSIC MATERIAL WilliamE. Flood,

sinszki, Fords, N.

notation of America,

lgetnchen, and Edward Koassiznorl to Catalin Cora corporation ofDelaware No Drawing. Application January 13,1945, Serial N0. 572,752 7This invention relates to cellulosic material impregnated with apolyvinyl resin and the method of making the impregnated product.

Claims. (Cl. 154-138) The invention is particularly useful in connectionwith the formationof paper containing polystyrene as the impregnatingmaterial and will be illustrated in detail by description in connectionwith this product.

When it is attempted to impregnate paper directly with polystyrene influid condition or emulsified in the best manner possible, adequatepenetration is not obtained. The polystyrene becomes concentrated on thesurface of the paper and does not impregnate satisfactorily the interiorparts of the paper. As a result the paper so impregnated absorbsmoisture and tacted with water.

In an attempt to overcome this diflicult it ha been proposed to useagents to modify the electrical nature of the paper, as, for instance,to increase the difference of electrical potential between the paper andthe polystyrene resin. Thus there have been used certain mordants ormordant-like materials to modify the electrical charge on the cellulosefibers when suspended in water.

In general, the polystyrene treated paper heretofore available hasretainedthe disadvantages of excessive concentration ("riding) of theresin on the surface portions of the paper and general unevenness ofdistribution, resulting frequently in surface freckling, warping of theproduct on contact with water, or checking on standing.

The present invention provides a convenient and effective impregnationof paper to produce a product free from these disadvantages. Theinvention also makes possible commercial impregnation of paper in rollform with a large of paper to each other, and

vent, and then removing the said solvent subsequent to thepolymerization. In the preferred embodiment, the invention comprisesimpregnating paper in roll form with a solution of styrene, polymerizingthe,styrene in the paper in a closed vessel, unrolling the paper, andthen evaporating the solvent therefrom. In one embodiment, the inventioncomprises the additional steps of laminating individual sheets of paperor the like so impregnated by plying the sheets together after removalof the solvent, warming the composite to a temperature at least as highas the fiow point of the resin present in the sheets, consolidating thplied sheets under pressure at the elevated temperature, cooling thethus consolidated article to a temperature below the solidificationpoint of the resin present, and then releasing the pressure.

The polymerization of the monomer, before the evaporation of the solventused to introduce the monomer into the paper, not only serves thepurpose Of forming a thermoplastic polymer but also converts the monomerused to such form that it does not migrate to an objectionable extent tothe surface of the cellulosic material during the subsequent evaporationof the solvent. It will be'appreciated that a material dissolved in asolvent ordinarily migrates with the solvent to the surface of thearticle from which the solvent is being evaporated. If the dissolvedmaterial is non-volatile, it is then deposited at that surface, to forma crust or concentrated surface film. In the present instance, thepolymerization to the large molecules prevents this migration to anyobjectionable extent. As a result,

the impregnated and then polymerized material .sidered that thedecreased migration i due in large measure to the conversion of themonomer to a polymer of such very high molecular-weight, of the order of10,000 at least, that the large molecules do not move, by eithereapillarity or diffusion, through the paper orother cellulosic materialto an objectionable extent as the. solvent evaporates from the surfacethereof. This non-migration is favored also by the quick evaporation ofthe solvent, as will be described.

The paper used may be that made from rag, kraft, sulflte, or like stock.Preferably it is of pregnating solution into oxide, and dilaurylperoxide.

a relatively porous nature as compared to the averagerun of paper.

As the solvent used to dissolve the monomer before impregnation, theremay be used xylene,

toluene, ethyl acetate, dioxane or like known volatile solvents for themonomer or a,mixture of two or more of them. These solvents are known tobe inert, that is, non-polymerizing under the conditions of treatmentdescribed herein.

The monomer is dissolved in the selected solvent in proportion to giveabout as concentrated a solution as is free flowing and permit drainageof the excess solution from the impregnated cellulosic material afterthe polymerization is completed.

Thus there is used in the volatile solvent that is about as high aspossible without producing a viscous solution. There may be used, forexample, a solution containing 35 to 50 parts of styrene in 100 parts ofsolution. The higher the concentration of the monomer, the higherordinarily will be the concentration of polymer in the impregnatedproduct, if the impregnation is satisfactorily effected before thepolymerization. It is desired for many purposes to obtain and we doobtain by our method polymer concentrations in the finished product of25 to 50%, proportions here and elsewhere herein being expressed asparts by weight.

The restriction of the evaporation of solvent during the polymerizationis effected conventionally by use of a thick mass of the cellulosicmaterial in a closed vessel. Thus, paper in roll form is impregnated andsubjected to the polymerization, the impregnated roll being enclosed ina substantially vapor-tight vessel during the polymerization. The rollis preferably submerged at the time in an excess of the impregnatingsolution.

To improve the impregnation the roll of paper is preferably predried,subjected to a vacuum, and maintained in the treated evacuated conditionuntil the vacuum is broken by flowing the imthe evacuated vessel andaround and over the roll of paper. Impregnation is increased toadvantage by the application to the impregnating liquid of pressuresthat are substantial and preferably up to approximately 25 to 100 poundsor more to the square inch. In an alternative procedure, undried paperis immersed in benzene, toluene, xylene or like liquid, the liquiddistilled off so as to remove water from the paper, remaining liquid isdrawn oir, adhering liquid is evaporated in vacuo, and the paper thenimpregnated as described with styrene or like monomer.

The solution used to effect the initial impregnation includes not onlythe solvent and the monomer but preferably also a catalyst of thepolymerization. Thus, there may be used catalytic proportions of benzoylperoxide, butyl hydroper- The catalyst accelerates the polymerizationand results, when the polymerization is cor ipleted, in a polymer oflower average molecular weight than would be the case if thepolymerization were completed in the absence of the catalyst.

The temperature of polymerization in every case must be below the pointor objectionable decomposition of the selected cellulosic material. Atthe same time it should be sufllciently elevated to make the rate ofpolymerization satisfactory for commercial purposes. I Room temperature,for instance, is somewhat too slow practically. Ordinarily the range oftemperature is a concentration of styrene days, the exact about 50 to125 C. and for most purposes within the range 65 to 100 G. Since thepolymerization is strongly exothermic, the reaction is difficult tocontrol at temperatures much above 110 C. In small operations; however,and with effective means of moderating the reaction by cooling, thetemperature may range from those stated up to about 140 C.

The course of the polymerization is readily followed by withdrawingsamples of the excess of liquid from time to time or samples of theimpregnated paper and determining the molecular weight and degree ofpolymerization of the resin therein.

The time of the polymerization used in the present invention isordinarily that required to give substantially complete polymerizationor at least the conversion of the monomer to a nonvolatile polymer. Thistime requirement is ordinarily measured in days as, for instance, 5 to30 time required depending upon the temperature, proportion and kind ofcatalyst, and other conditions affecting in known manner the rate ofpolymerization of the monomer.

After completion of the polymerization, the molecular weight of theresulting polystyrene is about 10,000 to 120,000 and in commercialoperations usually 65,000 to 120,000.

As an aid in preventing the molecular weight of the polymer from eitherincreasin too little or too slowly or becoming objectionably large whenthe polymerization is completed, it is preferred ,to use not only thecatalyst or accelerator but also a relatively low temperature at thestart of the polymerization and follow this later by a somewhat highertemperature. Thus, there may be used 65 to C., then 80 to 100 C., andfinally 100 to 125 0., preferably 80 C. for 2 days, followed by atemperature of C. for 5 additional days, and then to C. for 3 days. Thelow temperature is used to keep the average molecular weight high or inthe commercial range and compensate in this regard for the presence ofthe catalyst.

After the impregnation of the paper and the polymerization, the roll isremoved from the polymerization vessel after cooling, as for instance,to well below the boiling point of the solvent at normal pressure andusually to room temperature.

While the paper is still wet with solvent, it is unrolled and theunabsorbed solvent and polymerized styrene are drained off.

The solvent is then removed from the unrolled paper. This removal iseffected conveniently by passage of the unrolled paper through a conventional drying chamber in which the paper is floated in a rising currentof steam heated air. While other methods of drying may be used, thisfloating of the paper through the chamber of warm air is particularlysatisfactory in avoiding excessive contact with parts of the mechanismand giving a rapid rate of evaporation of the solvent. This leaves thesheet and even the fibers themselves impregnated with styrenepolymerized in situ but the surface of the sheet does not carry .anexcessive layer of the polymerized styrene.

The sheeted product can readily be stored without danger of undesiredadherence between adjacent layers.

The dried paper venient storage. structural sheet.

In this plyingtogether, the polystyrene impregnated paper is cut intostrips ofthe desired dimensions', then assembled together face to back,to give the proper number for the ultimate thickness desired for theconsolidated product, the

may then be rerolled for con- It is used in making a plied assembly isplaced in a press provided with means for heating the assembly, thetemperature is raised to a point at least equal to'and suitablymoderately above the flowing point of the thermoplastic polymer presentin the paper, and

, pressure is applied to force the sheets together and consolidate themwhile the elevated temperature is maintained. 'After the consolidationis effected, the composite is cooled to below the softening point of'theresin and the pressure of consolidation then released.

In-this manner there has been made a structural material that is strong,uniform, substantially water impenetrable, non-checking on aging,

free from warping on exposure to moisture even though out or machined toexpose fresh surfaces, and, in moderate thicknesses, is resiliently'fiexible.

- give and, therefore, non-brittleness.

If desired, coloring material, plasticizers or other modifiersconventionally used in the polyvinyl plastics industry, and partingcompounds and lubricants may be incorporated into the impregnatingsolution originally introduced into the paper. Thus, there may be useddyes and pigments, and stearic acid or calcium stearate in theproportion of 0.1% to 1%.

In place of the paper used as the base material in the aboveillustration of the invention, there may be used cotton fabric, woodfiber insulating boards, or other cellulosic material that hassubstantial porosity so that it may be entered by the impregnatingsolution of the monomer in a volatile solvent. The cellulosic materialselected in place of the paper is suitably treated in the mannerdescribed for paper, including the preliminary drying, evacuation andimpregnation under pressure. In any case the polymerizationis effectedwhile restricting evaporation of the solvent.

In place of the styrene used as a monomer and converted by thepolymerization to polystyrene, there may be used other monomer that isheat polymerizable to a thermoplastic resin. ,Examples of such monomersthat may be substituted pound for pound for the styrene are methylmethacrylate, vinyl chloride, vinyl acetate, vinylchloracetate, andmixtures with each other or with styrene. The temperature ofpolymerization of the impregnated cellulosic product must be below thetemperature of decomposition of cellulose and may be the temperatureconventionally used in curing the selected monomer, the time period ofpolymerization being also conventional for the temperature chosen.,

The invention will be further illustrated by description in connectionwith the following specific examples.

. Example 1 A inch roll of rag paper '(Hurlburt paper #499) containing16% of zinc sulfide was impregnated with a solution containing styrenemonomer N99 in the proportion of 35 parts, xylene as S01? vent 65 parts,and t-butyl hydroperoxide aspolymerization catalyst 0.07 part. The rollas'used had a 2.5 inch core, that is, a cylindrical free space of thatdiameter extending through the center of the roll. Before impregnation,the roll was dried'ior 16 hours in a steam jacketed autoclave carrying15 pounds of steam in the jacket, evacuation being applied for 6 hours.

The roll so dried was then immersed in approximately 50 pounds of thestyrene solution described. The autoclave was closed and polymerizationthen eflected at 75 to 78 C. for 18 days.

The roll was then cooled to room temperature, removed from theautoclave, unwound, and passed, as unwound, through a floating paperdrier at such speed that the paper at any given point was in the drierfor about 5 minutes, a temperature of 275 F. being maintained in thewarm air chamber through which the paper was floated.

Example 2 The procedure described in the immediately preceding examplewas followed except that the vacuum in the autoclave was broken byadmitting the solution of styrenein xylene and pressure was'then appliedabove the surface of the solution in the autoclave, this pressureimproving the rate of penetration of the impregnated solution.Proceeding as described, there has been obtained impregnated papercontaining satisfactorily uniform proportions of the polymer in sheetsfrom all parts of the roll of paper originally immersed and sufiicientpolymer to flow together during the use of the impregnated paper inlaminating the sheets together. Thus there have been obtainedimpregnated rolls which differ by less than 3.5% in the resin content ofthe paper from the exposed outer or inner. surface of the hollow rolland the part of the paper which' was midway between the exposed innerand outer surfaces during the impregnation.

Example 3 .are consolidated into a structural sheet in the followingmanner.

Ten sheets of the paper made as described in Examples 1 or 2 are laidface to back upon each other and the assembly warmed to 320 F. andcompresed at 1,000 pounds to the square inch for a period of 30 minutes.The product so made and then cooled had a water absorption of 9% onimmersion in liquid water and was substantially free from warping andchecking.

4 surface treated with a solution of prepolymerized styrene, that is,polystyrene dissolved in five times its weight of xylene. The thuscoated paper was dried. It was then plied together in 10 sheet thicknessand consolidated at 320 F. and pressure of 1,000 pounds to the squareinch for 30 minutes. was 6.85%.

It will be understood that it is intended to cover all changes andmodifications of the examples of the invention herein chosen for thepurpose of illustration which do. not constitute depiartures from thespirit and scope of the invent on.

What. is claimed is: Y

1. The method of impregnating cellulosic material which comprisesapplyingthereto styrene monomer and an inert volatile liquid that is asolvent for the said monomer and readily volatile,

In this instance the water absorption 7 preventing loss of volatilesolvent, and then evaporating the volatile solvent from the product.

2. The method of impregnating paperwhich comprises drying the paperintroducing into the paper in roll form a solution of styrene and apolymerization catalyst in solution in an inert volatile solvent,warming the thus treated roll of paper to cause polymerization of thestyrene to a thermoplastic resin, continuing the warming until the resincomes tohave a molecular weight of at least 10,000, restrictingevaporation so as to prevent substantial loss of volatile solvent duringthis polymerization, unwinding the roll of paper, and then evaporatingvolatile solvent from the impregnated paper.

3. The method described in claim 2, the proportion of volatile solventbeing in excess of the proportion of styrene and the roll of paper beingcooled before being unwound.

4. The method described in claim 2, the catalyst of polymerization beingan organic peroxide.

5. In making a resin impregnated and bonded product, the method whichcomprises forming an impregnated sheet as described in claim 1, plyingtogether a plurality of such sheets in face to REFERENCES CITED Thefollowing references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 2,147,824 Webb Feb. 21, 19392,013,865 Sloan Sept. 10, 1935 1,953,083 Lawson Apr. 3, 1934 2,365,646New Dec. 19, 1944 OTHER REFERENCES Styrene," article published inPlastics, July 1942, page 233.

